https://sketchviz.com/@danielhenrymantilla/d5870819ccfd2d8c66146611e7d3a953

My Diagram.dot

# http://www.graphviz.org/content/cluster

digraph G {
    graph [fontname = "courier"];
    node [fontname = "courier", shape=box];
    edge [fontname = "courier"];

    compound=true;
    
    subgraph cluster_a {
        style = dashed
        label = "background (ffi) thread"
        
        a [label = "call_js_cb(...args)"]
        d [label = "ret_value"]
    }
    
    subgraph cluster_b {
        style = dashed
        label = "js thread"
        b [
            label = "js stuff\n                            "
            style = invis
        ]
        c [label = "enqueued_cb_call(ret_sender, ...args)"]
        c2[label = "ret_sender(ret_value)"]
    }
    a -> b [
        lhead = cluster_b
        label = "\
 enqueue call for next js tick\l\
 with \l\
  ret_sender = (x) => channel.send(x)\l\
 being added to the args\l\
 by safer-ffi.\l\
 Scheduled call:\l
 real_js_call(ret_sender, ...args)\n\
"
        constraint = false
    ]
    b -> c [
        label = " eventually:\ntick()"
        style = dotted
    ]
    b -> d [style = invis]
    c -> c2 [
        label = " call completes"
    ]
    c2 -> d [label = "channel.send(ret_value)"]
    
    a -> d [
        style = dotted
        label = " wait for return value in channel"
    ]
}